HST is a combination of a hydraulic pump and a hydraulic motor. Consequently, by changing the tilt angle of a swash plate in the hydraulic pump, and by changing the discharge amount in a range from zero to a maximum discharge amount, the rotational velocity of the hydraulic motor changes. A vehicle can thus continuously change speeds from a stopped state to a maximum forward or reverse speed.
Structures that comprise a single swash plate, a cylinder block, and a plurality of pistons that are housed on only one side of the cylinder block are often used as HST hydraulic pumps or hydraulic motors.
However, the size of the HST hydraulic pump or the hydraulic motor becomes large when a high volume is needed in the HST hydraulic pump or the hydraulic motor, respectively. In this case, a large space for mounting the HST to a vehicle is required, and this is detrimental to efficiency and cost.
An opposing type swash plate hydraulic pump or motor comprising not one swash plate, but instead a pair of swash plates opposing each other, has been proposed in JP 50-115304 A as a way to make it possible to reduce the size of a hydraulic pump or a hydraulic motor.